Group 15 Elements (Nitrogen Family) - Practice Questions & MCQ

Electronic configuration
The valence shell electronic configuration of these elements is ns²np³. The s orbital in these elements is completely filled and p orbitals are half-filled, making their electronic configuration extra stable.

Atomic and Ionic Radii
Covalent and ionic (in a particular state) radii increase in size down the group. There is a considerable increase in covalent radius from N to P. However, from As to Bi only a small increase in covalent radius is observed. This is due to the presence of completely filled d and/or f orbitals in heavier members.

Ionisation Enthalpy
Ionisation enthalpy decreases down the group due to gradual increase in atomic size. Because of the extra stable half-filled p orbitals electronic configuration and smaller size, the ionisation enthalpy of the group 15 elements is much greater than that of group 14 elements in the corresponding periods. The order of successive ionisation enthalpies, as expected is .

Electronegativity
The electronegativity value, in general, decreases down the group with increasing atomic size. However, amongst the heavier elements, the difference is not that much pronounced.

Reactivity towards hydrogen
All the elements of Group 15 form hydrides of the type EH₃ where E = N, P, As, Sb or Bi. The hydrides show a regular gradation in their properties. The stability of hydrides decreases from NH₃ to BiH₃ which can be observed from their bond dissociation enthalpy. Consequently, the reducing character of the hydrides increases. Ammonia is only a mild reducing agent while BiH₃ is the strongest reducing agent amongst all the hydrides. Basicity also decreases in the order NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃. Due to high electronegativity and small size of nitrogen, NH₃ exhibits hydrogen bonding in solid as well as the liquid state. Because of this, it has higher melting and boiling points than that of PH₃.

Reactivity towards oxygen
All these elements form two types of oxides: E₂O₃ and E₂O₅. The oxide in the higher oxidation state of the element is more acidic than that of lower oxidation state. Their acidic character decreases down the group. The oxides of the type E₂O₃ of nitrogen and phosphorus are purely acidic, that of arsenic and antimony amphoteric and those of bismuth predominantly basic.

Reactivity towards halogens
These elements react to form two series of halides: EX₃ and EX₅. Nitrogen does not form pentahalide due to non-availability of the d orbitals in its valence shell. Pentahalides are more covalent than trihalides. This is due to the fact that in pentahalides +5 oxidation state exists while in the case of trihalides +3 oxidation state exists. Since elements in +5 oxidation state will have more polarising power than in +3 oxidation state, the covalent character of bonds is more in pentahalides. All the trihalides of these elements except those of nitrogen are stable. In case of nitrogen, only NF₃ is known to be stable. Trihalides except BiF₃ are predominantly covalent in nature.

Reactivity towards metals
All these elements react with metals to form their binary compounds exhibiting –3 oxidation state, such as, Ca₃N₂ (calcium nitride) Ca₃P₂ (calcium phosphide), Na₃As (sodium arsenide), Zn₃Sb₂ (zinc antimonide) and Mg₃Bi₂ (magnesium bismuthide).

Anomalous properties of nitrogen
Nitrogen differs from the rest of the members of this group due to its small size, high electronegativity, high ionisation enthalpy and non-availability of d orbitals. Nitrogen has unique ability to form pπ-pπ multiple bonds with itself and with other elements having small size and high electronegativity (e.g., C, O). Heavier elements of this group seldom form pπ-pπ bonds as their atomic orbitals are so large and diffused that they cannot have effective overlapping. Thus, nitrogen exists as a diatomic molecule with a triple bond (one sigma and two pi) between the two atoms. Consequently, its bond enthalpy (941.4 kJ mol^–1) is very high.

On the contrary, phosphorus, arsenic and antimony form single bonds as P–P, As–As and Sb–Sb while bismuth forms metallic bonds in elemental state. However, the single N–N bond is weaker than the single P–P bond because of high interelectronic repulsion of the non-bonding electrons, owing to the small bond length. As a result the catenation tendency is weaker in nitrogen. Another factor which affects the chemistry of nitrogen is the absence of d orbitals in its valence shell. Besides restricting its covalency to four, nitrogen cannot form dπ –pπ bond as the heavier elements can e.g., R₃P = O or R₃P = CH₂ (R = alkyl group). Phosphorus and arsenic can form dπ – dπ bond also with transition metals when their compounds like P(C₂H₅)₃ and As(C₆H₅)₃ act as ligands.